C.G. Hannah, J.W. Loder, B.D. Petrie and J.A. ShoreAs a cooperative activity of U.S. GLOBEC and Canada GLOBEC, the structure, origin and implications of past decadal-scale hydrographic variability in the Scotian Shelf and Gulf of Maine region are being examined. Primary focus is presently on the 1960s when temperature and salinity values on the shelf were below average, associated with an increased contribution of Labrador Water to Slope Water moving onto the shelf at depth (Petrie and Drinkwater 1993).
In Phase I of the U.S. GLOBEC Georges Bank Program, 3-d composite seasonal temperature, salinity and density fields were obtained for years representing the cold 1960s and warm 1970s, using the Emerald Basin temperature time series as an index. These fields indicated that the largest hydrographic changes occurred along the shelf edge extending to the Middle Atlantic Bight and in deep channels and basins on the shelf. Geostrophic estimates indicated southwestward transport increases of 1-2 Sv along the shelf edge and upper slope during the cold 1960s, consistent with the northern origin proposed by Petrie and Drinkwater. The geostrophic calculations indicated that circulation changes elsewhere on the shelf were relatively small due to the offsetting contributions of the temperature and salinity changes to density.
Additional estimates of transport and circulation changes have been obtained using the composite 3-d density fields for winter and the 3-d finite-element model QUODDY (Lynch et al. 1996) run in diagnostic mode with baroclinic and tidal forcing. The model solutions confirm that transport along the shelf edge was stronger by about 1 Sv during the cold 1960s, and also indicate an increased throughflow of 0.1-0.3 Sv over the Scotian Shelf and around the Gulf of Maine and Georges Bank during this period. The changes on Georges Bank consisted of intensification and broadening (e.g. onto the Northeast Peak cod/haddock spawning grounds) of the clockwise gyre, and were associated with increased barotropic flow generated upstream rather than with the local density field.
Other activities underway include:
Collectively, the analyses to date indicate that the northwestern Atlantic shelf/slope region between the Grand Bank and Middle Atlantic Bight has some of the strongest decadal-scale ocean temperature and salinity variability in the North Atlantic. The variability appears to be associated with larger-scale oceanographic and atmospheric changes (e.g. Levitus 1989; Kushnir 1994), but is of the opposite sign to that over most of the North Atlantic, pointing to a major influence from variable equatorward advection in the subpolar western boundary current.
Future work in Phase II of the U.S. GLOBEC Georges Bank Program will include prognostic modeling (short-term prognostic "refinements") of the circulation changes between the 1960s and 1970s periods, and investigation of biological implications in collaboration with GLOBEC colleagues.
Levitus, S. 1989. Interpentadal variability of temperature and salinity at intermediate depths of the North Atlantic Ocean, 1970-74 versus 1955-59. J. Geophys. Res. 94, 6091-6131.
Lynch, D.R., J.T.C. Ip, C.E. Naimie and F.E. Werner. 1996. Comprehensive circulation model with application to the Gulf of Maine. Cont. Shelf Res. 16, 875-906.
Petrie, B.D. and K. Drinkwater. 1993. Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945-1990. J. Geophys. Res. 98, 20079-20089.